1. Field of the Invention
The present invention generally relates to optical communication systems. More particularly, some example embodiments relate to demodulators for phase shift keyed signals.
2. Related Technology
Communication technology has transformed our world. As the amount of information communicated over networks has increased, high speed transmission has become ever more critical. High speed communications often rely on the presence of high bandwidth capacity links between network nodes. For optical links, an optoelectronic module such as a transceiver or transponder module at one network node converts electrical data into optical data for transmission on the optical channel. At the other network node, another transceiver module receives the optical signal, and converts the signal into an electrical signal. Transceivers are equipped with transmit and receive channels, such that bi-directional communication is possible.
Presently, standards are being developed for optical links at a speed of 40 Gigabits per second (sometimes abbreviated as “40G”). In fact, the Institute for Electrical and Electronics Engineers, Inc. (often referred to as “IEEE” for short), a leading professional association in the art of networking technologies, has recently voted that the next generation of Ethernet technology will provide support for 40 Gigabit Ethernet as well as 100 Gigabit Ethernet, and has established several task forces to develop appropriate standards that are yet under development.
Currently, 40G Single Mode Fiber (“SMF”) and Multi-Mode Fiber (“MMF”) standards for Ethernet optical link applications are under development. The signals sent in a 40G fiber will be required to be modulated using phase shift keying (PSK), differential phase shift keying (DPSK), or differential quadrature phase shift keying (DQPSK) on the transmit side, and demodulated at the receive side.
A PSK optical signal typically includes a return-to-zero (RZ) signal having a series of relatively high intensity pulses separated by low intensity regions. For a DPSK optical signal, the phase difference between adjacent pulses may encode information. For example, in some DPSK encoding schemes, a phase difference of π encodes a one bit whereas a phase difference of zero or 2π encodes a zero bit. For a DQPSK optical signal, the phase differences may be, for instance, 0 (or 2π), π/2, π, and 3π/2 corresponding to data bits “00”, “01”, “11”, and “00” respectively.
Demodulation of a PSK signal includes converting the phase information encoded in the pulses into amplitude modulation such that the data can be detected by means of a photodiode or other optical sensor. In a conventional demodulator, this is accomplished by means of a delay line interferometer (“DLI”), such as a Mach-Zehnder interferometer or Michelson interferometer. A DLI operates by dividing an input signal into first and second signals. The first and second signals travel along paths of different lengths and are then rejoined into one or more output signals. The difference in path length is chosen such that upon recombining, the first and second signals will constructively and/or destructively interfere with one another depending on the phase difference between adjacent pulses.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.